Radio communication devices, such as portable telephones, require a stable operating frequency provided by a local oscillator. Typically, the frequency of a local oscillator is generated by a voltage controlled oscillator (VCO). The operating frequency of voltage controlled oscillators used in these portable telephones is strongly dependent on parts tolerances and manufacturing techniques.
In particular, the etching variations of microstrips defined on circuit boards of VCOs can cause large frequency shifts of up to 4%. Some prior art methods to adjust the operating frequency of VCOs have included; deforming coils, using trimmable capacitors and using variable inductors. These methods have the disadvantage of having the potential to change frequency in the field, either through a mechanical shock or through intentional physical adjustment.
Other more permanent prior art methods to adjust the operating frequency of VCOs have used cutting techniques to remove conductive material from the circuit traces or specially provided large area capacitors designed to be mechanically trimmed. The actual cutting action was provided by laser trimming, sand abrasion, or even using a Dremel.TM. tool. All of the above techniques use active trimming to achieve a desired frequency and require additional production machinery. In particular, laser trimming systems which are commonly used are very expensive. The mechanical trimming methods have high labor costs and are not repeatable.
There is a need for a VCO tuning method that is simple and passive. There is also a need for a tuning method that saves space in a VCO and does not introduce contaminants on top of the circuit board. In addition, there is a need for a tuning method that does not require expensive production machinery and provides high yield.